If the prop is turning in a clockwise direction (in forward) when viewed from astern then the bow swinging to port may be because the clearance between the top of the propeller and the bottom of the boat is insufficient and the prop is cavitating at the top. That might cause the boat to veer to port at higher speeds but be less noticeable at slow speeds because there would be no cavitation. It would likely be less noticeable when going astern because the revs are usually much lower (no cavitation).

That is my guess......

Thinking about it some more, the fact that you went from a three blade to a two blade may also contribute to the new cavitation. With two blades, the top blade may be cavitating while the bottom blade is biting introducing the prop walk. Of course with the three blade, whenever there was a blade straight down there wasn't one near the hull so cavitation was minimized.

There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217

If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps better than most. A small sailing craft is not only beautiful, it is seductive and full of strange promise and the hint of trouble.

Conceivably it could be cavitation However the manufacturer, on viewing the photos showing the large space from the blade end to the hull, gave up on that possibility. Yet this prop has an extremely efficient blade and is not shaped like most folding blades - so perhaps it is cavitating due to that. So I am back to my original question on this thread. Would a reduction of 1" pitch help the problem? Let's assume both cases: that it is cavitating and that it is not cavitating. Or would a reduction in diameter be better? That was the thrust of my original question...

One more note... If I turn the rudder over to the other side... to the right, at a certain point, the force changes and starts pulling the wheel the other way.... indicating that the "not so balanced" rudder is being pushed much harder and unequally by water force that it was formerly. In other words the water flow from this prop is not equal on both sides of the rudder (for whatever reason, perhaps cavitating???) , and the rudder becomes unbalanced. So... a smaller pitch or a smaller diameter... or will it make no difference at a give speed.

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Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Another clue that may be key: even with the strong pull to port, the rudder is aligned straight foreward and aft. To me this implies a right hand paddle wheel effect rather than an imbalanced rudder under power.

JB2: keep in mind that this new design prop has almost no reverse prop walk. It is only a "problem" at higher speeds forward. All in all, a super 2 blade folding prop that eliminates your reverse prop walk problem

It still sounds to me like it is cavitating and not getting enough clean flow at the top dead centre position when moving ahead. I would try reducing the pitch (if that is easy and inexpensive) first. The boat may go a little slower but it also may solve the problem.

There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217

So I am back to my original question on this thread. Would a reduction of 1" pitch help the problem? Let's assume both cases: that it is cavitating and that it is not cavitating. Or would a reduction in diameter be better? That was the thrust of my original question...

I don't think it will, I changed my Maxprop pitch to get the max revs up and it made no difference the effect on the rudder at max power. If anything I think the higher rpm produces more effect. I somehow doubt the cavitation effect in this, as the cavitation only occurs close to the propeller and you say your rudder and hull are well clear. I think it is more to do with the difference between the boat speed flow and the prop wake flow at the rudder. If you shut down the engine at max speed, the effect goes away, so it's certainly either prop walk or prop wake effect on the rudder.

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